The present invention concerns an ionization chamber, an activity measuring channel for a xcex2 radiation gas emitter, which can for example be a tritium detection chamber, and an implementation process for this.
An ionization chamber tritium channel such as described in the document in reference [1] at the end of the description, is used to measure the activity of a xcex2 radiation gas emitter in a given gaseous environment, for example in a glove box, a laboratory ventilation circuit or furthermore a stack control in a nuclear building. The ionization chamber which is immersed directly in the environment to be controlled, provides a current in proportion to the activity to be quantified. Process electronics facilitate the measurement of amperage between 10xe2x88x9214 and 10xe2x88x928 A.
Such a measuring channel is made up of an ionization chamber, which plays the role of detector, a pre-amplifier, signal processing electronics, and a link cable between the pre-amplifier and the processing electronics.
The ionization chambers of today""s technology are of two types:
the massif anode and cathode chambersxe2x80x94a chamber of this type illustrated in FIG. 1 comprises a central electrode 10 in nickel brass forming an anode, a shell 11 in brass forming a cathode, a filler hole 12, an end 13 for adapting a pre-amplifier, a guard ring 14 in nickel brass, insulators 15 in polystyrene and O-rings 16.
Such chambers are used for the measurement of low activity, for example below 5,000 Admissible Contamination Limit. These are generally of large size, for example about 10,000 cm3.
Anode and/or light cathode chambersxe2x80x94a chamber of this type illustrated in FIG. 2 comprises removable electrodes 20 and 21, in practice a central anode 20 and a cathode 21 formed by a brass, copper or aluminium grid and is fitted with a filler hole 22. In this case it is set out in a shell 23 in steel, with two openings 24 and 25 respectively for filling and draining. Furthermore it comprises an O-ring 26, a guard ring 27, insulators 28 in polystyrene, an outlet 29 to a high voltage and an outlet 30 to a pre-amplifier.
Such chambers are dedicated to measuring higher activities (glove box, or even certain manufacturing processes). They are generally small-volume chambers, for example 100 cm3. The use of light electrodes for the measurement of intense activity is aimed at limiting the surface contamination of electrodes which generates considerable background noise.
In the channels of today""s technology, a pre-amplifier assembled directly on an ionization chamber facilitates amplifying the current which can be very low up to a satisfactory level so that it can be transmitted through the connector cable to the processing electronics.
Two methods are currently used for the signal process electronics to quantify the current due to ions produced by the xcex2 radiation and picked up on the electrodes of the ionization chamber:
capacitor load (load quantifier)
high resistance voltage read on the terminals in which the generated current circulates.
Electronics of the first type are acknowledged to be more accurate because drift over a period of time is lower. Electronics of the second type are less complicated to implement and less expensive.
What is more the signal processing electronics must have considerable measuring scope (from 10xe2x88x9214 A to 10xe2x88x928 A, or from 10xe2x88x9212 A to 10xe2x88x926 A), which means an automatic rating change-over.
Such measuring channels of today""s technology have numerous drawbacks:
the prohibitive cost of electronic systems allowing measurement of very low currents,
the important background noise of light electrode chambers,
in certain cases these chambers are inoperative after one single measurement,
the impossibility of decontamination by baking (400xc2x0 C.) under vacuum of existing ionization chambers,
the lack of heater systems in all chambers, in particular those of less than 100 cm3 in volumexe2x80x94measurements are in fact more stable when the atmosphere in the vicinity of the chamber can be heated (stabilization at a given temperature, convection stirring, impact on hygrometry, etc.),
the need to replace a complete chamber+base+connector unit in case of breakdown or after important contamination which is very expensive, as today""s chambers are in fact welded to their connection systems and to their mechanical base so as to limit noise generated by the contact resistors,
the need to have recourse to guard rings in order to correctly delimit lines of electric fields in the ionization chamber, as certain pieces of the framework of today""s chambers (excluding cathode, anode and connector)are in fact made in conductive material,
the absence of leaktightness of the mechanical base which is the interface between the ionization chamber and the connector,
generation of a not inconsiderable weight (today about 1 kilo) of contaminated waste at the time of replacing a complete detector (chamber+base+connector),
the multiplication of measuring channels (250 measurement lines constantly operational in a base nuclear installation treating tritium) which involve important installation costsxe2x80x94a complete measuring channel (detector+process electronics+cable) costs between 72 kF (about 14 kF for a detector of 100 cm3 and about 58 kF for the electronics) plus about 126 kF (68 kF for a detector of 10,000 cm3 and about 58 kF for the electronics).
The purpose of the present invention is to overcome the shortcomings of first generation devices.
The present invention concerns an ionization device cylindrical in shape comprising an ionization chamber with an anode made up of a central rod in current-carrying material, and a cathode in current-carrying material around the said anode, both connected to two elements of a mechanical base of the said chamber, two cylindrical end shields in non-magnetic and insulating material, centred on the anode and arranged at right angles to this at both ends.
As the cathode is made up of a spooled wire on the outer rim of these two end pieces, typified in that the base is removable, the lower extremity of the anode and the two ends of the wire making up the cathode connected to pin plugs arranged on the lower end plate, being suitable for insertion in the sockets arranged on a contact holder unit on the base, and in that the end shields are provided with openings.
In a convenient realization method, the base is fitted in its two extremitiesxe2x80x94upper and lower respectivelyxe2x80x94with a contact holder unit and connector, the said unit comprising the sockets arranged on the lower end shield and connected by conducting wires to connector lugs. A cylindrical protective shield for the chamber is fixed on the upper part of the base. Conveniently the anode is in stainless steel, the two end shields are in Teflon (PTFE), in ceramics or a mixed material (ceramics+Teflon), and the cathode comprises a platinum wire, for example 0.05 mm in diameter.
The present invention equally concerns an activity measuring channel for a xcex2 radiation gas emitter, comprising such an ionization chamber, a pre-amplification unit assembled just behind the ionization chamber, offset signal process electronics and a connector cable between the pre-amplification unit and the process electronics. Conveniently the pre-amplification unit makes an analog to digital conversion.
The measuring channel can, for example, serve as a tritium measuring channel.
The present invention also concerns an implementation process for this ionization chamber such as provided by one of the three following variants, in which a heat current is circulated in the cathode:
during the measurement so as to create movements of convection of the gaseous mixture to be measured,
during the measurement so as to stabilize both the sensor temperature and impact on the hygrometry of the gaseous mixture,
at the time of decontamination vacuum baking, as the temperature is above 400xc2x0 C.